Reception device and radio communication system using the same

ABSTRACT

A reception device  81 ) has reception antennas ( 11 - 1  to  11 - n ) for receiving signals transmitted from a plurality of transmission antennas and outputting reception signals. A channel estimation device ( 12 ) estimates a channel between the antennas and outputs a channel estimation value H. A path selection device ( 13 ) inputs the channel estimation value H and calculates the channel correlation value between the transmission/reception antennas from the channel estimation value H. If the calculation result is smaller than a reference value, the path selection device ( 13 ) outputs a path selection signal selecting a path of a high power with a higher priority. On the other hand, if the calculation result is greater than the reference value, the path selection device ( 13 ) outputs a path selection signal selecting, with a higher priority, a path having a lower correlation value with a path detected by the other reception antenna. Despread devices ( 14 - 1  to  14 - kn ) input the reception signal and the path selection signal and output a despread signal. A demodulation device ( 15 ) inputs a despread signal and outputs a reproduction series.

TECHNICAL FIELD

The present invention relates to a reception device and a radiocommunication system using the reception device and, in particular, to aspread-spectrum radio communication system using a plurality ofantennas.

BACKGROUND ART

In spread-spectrum radio communication systems, the reception quality isimproved by separating or combining a plurality of paths identified by areception device. When a transmission device and a reception device usea plurality of antennas, the reception quality can be improved byselecting and combining paths using the same technique.

With reference to FIG. 1, a spread-spectrum radio communication systemdisclosed in Japanese Unexamined Patent Application Publication No.2001-230702 is described. Here, the number m of transmission antennas is2 and the number n of reception antennas is 2. In a transmission device5, the same spreading code is used for all transmission antennas 21-1and 21-2. Additionally, a delay time in the path of a reception antenna11-1 is identical to that of a reception antenna 11-2. The propagationpath for each antenna includes two paths, as shown in FIG. 2.Furthermore, a reception device 4 has one de-spreader unit.

FIG. 3 illustrates an exemplary configuration of the reception device 4.Reception signals r₁(t) and r₂(t) received by the respective firstantenna 11-1 and the second antenna 11-2 are expressed by the followingequations:

r₁(t) = h¹¹ ⁻ ¹c(t − τ₁)b₁(t − τ₁) + h¹¹ ⁻ ²c(t − τ₂)b₁(t − τ₂) + h¹² ⁻ ¹c(t − τ₁)b₂(t − τ₁) + h¹² ⁻ ²c(t − τ₂)b₂(t − τ₂) + n₁(t)r₂(t) = h²¹ ⁻ ¹c(t − τ₁)b₁(t − τ₁) + h²¹ ⁻ ²c(t − τ₂)b₁(t − τ₂) + h²² ⁻ ¹c(t − τ₁)b₂(t − τ₁) + h²² ⁻ ²c(t − τ₂)b₂(t − τ₂) + n₂(t)where the fluctuation of the propagation path is sufficiently delayedwith respect to the fluctuation of the transmission signal. τ₁ and τ₂represent paths. c(t−τ₁), b_(m)(t−τ₁), n_(n)(t) are a spreading code, adata signal from an m-th transmission antenna 21-m, and a noise signalof an n-th reception antenna 11-n, respectively. h_(nm-1) represents afirst path between the m-th transmission antenna 21-m and the n-threception antenna 11-n. As described above, m=n=2.

A channel estimation unit 12 carries out channel estimation usingreceived signals and outputs the obtained channel estimation value H. Ingeneral, a spread-spectrum communication system selects the paths indescending order of power. Accordingly, a maximum-power-path selectionunit 41 selects a path having the largest power and outputs a maximumpath signal X_(max). Here, it is assumed that a preceding path haslarger power. In this case, de-spreader units 14-1 and 14-2 de-spreadbased on the maximum path signal in synchronization with the timing ofthe preceding path.

For the sake of simplicity, one received symbol is focused on. Ade-spreading signal y₁ of the first reception antenna 11-1 is given bythe following equation (1):

$\begin{matrix}\begin{matrix}{y_{1} = {\int_{\tau_{2}}{{c( {t - \tau_{1}} )}{r_{1}(t)}\ {\mathbb{d}t}}}} \\{= {{h_{11 - 1}b_{1}} + {h_{12 - 1}b_{2}} + {R( {\tau_{1} - \tau_{2}} ){\sum\limits_{m = 1}^{2}{h_{{1m} - 2}{b_{m}( {t - \tau_{2}} )}}}} + n_{1}^{\prime}}}\end{matrix} & (1)\end{matrix}$where T_(s) is one symbol time, R is an auto-correlation function of aspreading code, and n′₁ is Gaussian noise.

As shown in FIG. 4, a widely used auto-correlation function of aspreading code exhibits a sharp peak at a time difference of zero andexhibits a sufficiently small value at a time difference of othervalues. Therefore, the second term of equation (1) can be ignored here.

As a result, the de-spreading signal y₁ is expressed as:y ₁ =h ₁₁₋₁ b ₁ +h ₁₂₋₁ b ₂ +n ₁.

Similarly, a de-spreading signal y₂ of the second reception antenna 11-2is given by the following equation:y ₂ =h ₂₁₋₁ b ₁ +h ₂₂₋₁ b ₂ +n ₂.

A demodulation unit 15 inputs the de-spreading signals and carries outdemodulation to reproduce a data sequence. The de-spreading signal isgiven by the following equation (2) in vector representation:

$\begin{matrix}\begin{matrix}{Y = \begin{bmatrix}y_{1} \\y_{2}\end{bmatrix}} \\{= {{\begin{bmatrix}h_{11 - 1} & h_{12 - 1} \\h_{21 - 1} & h_{22 - 1}\end{bmatrix}\begin{bmatrix}b_{1} \\b_{2}\end{bmatrix}} + \begin{bmatrix}n_{1} \\n_{2}\end{bmatrix} + {Hb} + n}}\end{matrix} & (2)\end{matrix}$

The demodulation unit 15 reproduces a transmission sequence by, forexample, premultiplying the equation by a reverse matrix of the channelestimation value H, as follows:b ₀ =H ⁻¹ Y=b+H ⁻¹ n.

When all of the propagation paths are independent, a reverse matrix ofthe above-described channel matrix can be correctly obtained. However,if channel replies have correlation, a reverse matrix does not exist,thereby disabling reception.

For example, if a propagation path between the first transmissionantenna 21-1 and the first reception antenna 11-1 has completecorrelation with a propagation path between the first transmissionantenna 21-1 and the second reception antenna 11-2 and if a propagationpath between the second transmission antenna 21-2 and the firstreception antenna 11-1 has complete correlation with a propagation pathbetween the second transmission antenna and the second receptionantenna, h₁₁₋₁=h₂₁₋₁, h₁₁₋₂=h₂₁₋₂, h₁₂₋₁=h₂₂₋₁, and h₁₂₋₂=h₂₂₋₂. In thiscase, the channel estimation value H is given by the following equation(3):

$\begin{matrix}{H = \begin{bmatrix}h_{11 - 1} & h_{22 - 1} \\h_{11 - 1} & h_{22 - 1}\end{bmatrix}} & (3)\end{matrix}$

This results in |H|=0, and therefore, the inverse matrix does not exist.Consequently, it is hard to perform the demodulation process.

Accordingly, it is an object of the present invention to solve theabove-described problem and provide a reception device that achievessuperior communication quality even when propagation paths havecorrelation.

It is another object of the present invention to provide a radiocommunication system using the reception device.

DISCLOSURE OF INVENTION

A reception device according to the present invention includes n numberof reception antennas for receiving spread spectrum signals transmittedfrom a transmission device via m number of transmit antennas, where n isan integer greater than or equal to 2 and m is an integer greater thanor equal to 2. The reception device also includes a path selection unitfor selecting a path for de-spreading the signal received by thereception antennas and outputting a selection result as a path selectionsignal. The reception device further includes K_(n) number ofde-spreader units for de-spreading the signal received by the receptionantennas based on the path selection signal and outputting the de-spreadsignals, where K_(n) is an integer greater than or equal to 1, nrepresents an nth reception antenna, and 1≦n≦K_(n).

A radio communication system according to the present invention includesa reception device that comprises n number of reception antennas forreceiving spread spectrum signals transmitted from a transmission devicevia m number of transmit antennas, where m is an integer greater than orequal to 2 and n is an integer greater than or equal to 2. The receptiondevice further comprises a path selection unit for selecting a path forde-spreading the signal received by the reception antennas andoutputting a selection result as a path selection signal, and K_(n)number of de-spreader units for de-spreading the signal received by thereception antennas based on the path selection signal and outputting thedespreaded signals, where K_(n) is an integer greater than or equal to1, n represents an nth reception antenna, and 1≦n≦K_(n). The receptiondevice selects a path in accordance with a correlation value ofpropagation paths.

In a reception device according to the present invention, a receptionantenna receives a spread spectrum signal transmitted from atransmission device. A path selection unit inputs the spread spectrumsignal received by the reception antenna, selects a path forde-spreading the signal, and outputs a selection result as a pathselection signal. A de-spreader unit inputs the signal received by thereception antenna and the path selection signal, and outputs a de-spreadsignal based on the path selection signal.

A first path selection unit according to the present inventioncalculates a correlation value of propagation paths among the m numberof transmit antennas and the n number of reception antennas. If thecalculation result is smaller than a predetermined reference value, thepath selection unit independently selects K_(n) number of paths having alargest power at each of the n number of reception antennas. On theother hand, if the calculation result is greater than the predeterminedreference value, the path selection unit preferentially selects K_(n)number of paths having a low correlation with paths identified by theother reception antennas, at each of the n number of reception antennas.

A second path selection unit according to the present invention employsthe amplitude of a complex correlation value as the predeterminedreference value.

A third path selection unit according to the present inventiondetermines priorities of the n number of reception antennas and selectsP number of paths for de-spreading at each of the reception antennas inaccordance with the determined priorities, where P is an integer greaterthan or equal to 1 and 1≦P≦K_(n). The third path selection unit repeatsthe above-described selection operation until the number of pathsidentified by each of the reception antennas reaches K_(n).

A fourth path selection unit according to the present invention detectsa path having maximum power from among paths identified by each of the nnumber of reception antennas for each antenna when determiningpriorities of the reception antennas, and the path selection unitassigns a higher priority to a reception antenna having a detected pathof a higher power.

A fifth path selection unit according to the present invention detects apath having minimum power from among paths identified by each of the nnumber of reception antennas for each antenna when determiningpriorities of the reception antennas, and the path selection unitassigns a higher priority to a reception antenna having a path of alower detected power.

A sixth path selection unit according to the present invention detects apath having maximum power from among paths identified by each of the nnumber of reception antennas for each antenna when determiningpriorities of the reception antennas, and the path selection unitassigns a higher priority to a reception antenna having a path of alower detected power.

A seventh path selection unit according to the present invention detectsa path having minimum power from among paths identified by each of the nnumber of reception antennas for each antenna when determiningpriorities of the reception antennas, and the path selection unitassigns a higher priority to a reception antenna having a path of ahigher detected power.

A eighth path selection unit according to the present inventioncalculates average power of paths identified by each of the n number ofreception antennas for each antenna when determining priorities of thereception antennas, and the path selection unit assigns a higherpriority to a reception antenna having a higher average power value.

A ninth path selection unit according to the present inventioncalculates average power of paths identified by each of the n number ofreception antennas for each antenna when determining priorities of thereception antennas, and the path selection unit assigns a higherpriority to a reception antenna having a lower average power value.

A tenth path selection unit according to the present inventioncalculates the number of paths having a power exceeding P_(th) fromamong paths identified by each of the n number of reception antennas foreach antenna when determining priorities of the reception antennas,where P_(th) is any real value, and the path selection unit assigns ahigher priority to a reception antenna having more paths exceedingP_(th).

A eleventh path selection unit according to the present inventioncalculates the number of paths having a power exceeding P_(th) fromamong paths identified by each of the n number of reception antennas foreach antenna when determining priorities of the reception antennas,where P_(th) is any real value, and the path selection unit assigns ahigher priority to a reception antenna having less paths exceedingP_(th).

A twelfth path selection unit according to the present invention detectsa path having an earliest incoming time from among paths identified byeach of the n number of reception antennas for each antenna whendetermining priorities of the reception antennas, and the path selectionunit assigns a higher priority to a reception antenna having a detectedpath with an earlier incoming time.

A thirteenth path selection unit according to the present inventiondetects a path having an earliest incoming time from among pathsidentified by each of the n number of reception antennas for eachantenna when determining priorities of the reception antennas, and thepath selection unit assigns a higher priority to a reception antennahaving a detected path with a later incoming time.

A fourteenth path selection unit according to the present inventioncalculates an average delay time weighted by a power of a path amongpaths identified by each of the n number of reception antennas whendetermining priorities of the reception antennas, and the path selectionunit assigns a higher priority to a reception antenna having a shortercalculated average delay time.

A fifteenth path selection unit according to the present inventioncalculates an average delay time weighted by a power of a path amongpaths identified by each of the n number of reception antennas whendetermining priorities of the reception antennas, and the path selectionunit assigns a higher priority to a reception antenna having a longercalculated average delay time.

When determining priorities of the n number of reception antennas, asixteenth path selection unit according to the present inventiondetermines the priorities at random.

When determining the path for de-spreading at one of the receptionantennas, a seventeenth path selection unit according to the presentinvention determines a union of sets of incoming times of pathsidentified by each of the n number of reception antennas as a total setand determines a selected path set using path information about pathsselected for the reception antenna and the other reception antennas. Theseventeenth path selection unit also determines a difference between thetotal set and the selected path set as an unselected path set, anddetermines the path used for de-spreading from the unselected path set.

When determining the selected path set, an eighteenth path selectionunit according to the present invention determines samples from Fsamples before the incoming time of the path used for de-spreading atone of the reception antennas to B samples after the incoming time as apartial set of the selected path set, creates a union of the partial setof the selected path set for all of the n number of reception antennas,and determines the created set as the selected path set, where F is aninteger greater than or equal to 0 and B is an integer greater than orequal to 0.

When determining the path used for de-spreading from an unselected pathgroup, a nineteenth path selection unit according to the presentinvention selects a path having a highest reception power from theunselected path group.

When determining the path used for de-spreading from an unselected pathgroup, a twentieth path selection unit according to the presentinvention selects a path having an earliest incoming time from theunselected path group.

When determining the path used for de-spreading from an unselected pathgroup, a twenty-first path selection unit according to the presentinvention selects a path at random from the unselected path group.

A twenty-second path selection unit according to the present inventionselects the path used for de-spreading at random.

A spread-spectrum communication system according to the presentinvention forms a radio communication system by employing a receptiondevice that selects a path in accordance with a correlation value ofpropagation paths.

By employing the above-described structures and operations, thereception device and the radio communication system according to thepresent invention can achieve superior communication quality even whenpropagation paths have correlation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a known radio communicationsystem.

FIG. 2 illustrates an example of a propagation path model in the radiocommunication system shown in FIG. 1.

FIG. 3 is a block diagram illustrating the configuration of a receptiondevice shown in FIG. 1.

FIG. 4 illustrates an example of an auto-correlation function of aspreading code in the radio communication system shown in FIG. 1.

FIG. 5 is a block diagram illustrating the configuration of a receptiondevice according to a preferred embodiment of the present invention.

FIG. 6 is a block diagram illustrating the configuration of a receptiondevice having two reception antennas according to the present invention.

FIG. 7 is a block diagram illustrating the configuration of a receptiondevice having two reception antennas according to another embodiment ofthe present invention.

FIG. 8 illustrates an example of a propagation path model in theembodiment shown in FIG. 7.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 5 is a block diagram of a reception device according to anexemplary embodiment of the present invention. In FIG. 5, identicalelements to those of the reception device illustrated and described inrelation to FIG. 3 are designated by identical reference numerals. Areception device 1 includes n number of reception antennas 11-1 to 11-n,a channel estimation unit 12, a path selection unit 13, n number ofde-spreader units 14-1 to 14-kn, and a demodulation unit 15.

The reception device 1 receives spread spectrum signals transmitted froma transmission device (not shown) via the reception antennas 11-1 to11-n. The reception device 1 then outputs reception signals r₁ to r_(n).The channel estimation unit 12 carries out channel estimation using thereception signals r₁ to r_(n), and outputs an estimated channelinformation H.

The path selection unit 13 receives the channel information H as aninput and outputs a path selection signal X_(sel). At that time, thepath selection unit 13 first calculates a channel correlation value. Ifthe calculated correlation value is smaller than a predeterminedreference value, the path selection unit 13 outputs the path selectionsignal X_(sel) to preferentially select a path having a higher power ateach of the reception antennas 11-1 to 11-n. In contrast, if thecalculated correlation value is greater than the predetermined referencevalue, the path selection unit 13 outputs the path selection signalX_(sel) to preferentially select a path having a low correlation withother reception antennas.

The de-spreader units 14-1 to 14-kn de-spread the reception signals r₁to r_(n) in accordance with the path selection signal X_(sel) and outputde-spreading signals y_(n-k). The demodulation unit 15 inputs thede-spreading signals y_(n-k) to demodulate the signals into a datasequence, and outputs a reproduced signal b₀.

FIG. 6 is a block diagram illustrating the configuration of a receptiondevice including two reception antennas according to the presentinvention. As shown in FIG. 6, a reception device 2 includes tworeception antennas 11-1 and 11-2, which correspond to the number (two)of the transmission antennas of a transmission device (not shown).De-spreader units 14-1 and 14-2 are connected to the reception antennas11-1 and 11-2, respectively. All of the propagation paths between thetransmission antennas and the reception antennas comply with the pathmodel described with reference to FIG. 2. Furthermore, signalstransmitted from the transmission antennas have complete correlation.The paths have no correlation. Also, the transmission antennas have nocorrelation.

In the transmission device, the same spreading code is employed for allof the transmission antennas. Here, a reception signal is given by thefollowing equation (4):

$\begin{matrix}{{r_{n}(t)} = {{\sum{\sum{h_{{nm} - 1}{c( {t - \tau_{1}} )}{b_{m}( {t - \tau_{1}} )}}}} + {n_{n}(t)}}} & (4)\end{matrix}$

The channel estimation unit 12 carries out channel estimation using thereception signals and outputs channel information H. The path selectionunit 13 calculates a channel correlation using the channel informationH.

In general, a correlation coefficient for complex numbers is given bythe following formula (5).

$\begin{matrix}\frac{{ab}^{*}}{{a}{b}} & (5)\end{matrix}$

For example, to obtain a correlation coefficient between h₁₁₋₁ andh₁₂₋₁, since these paths have a complete correlation, that is,h₁₁₋₁=h₁₂₋₁, the correlation coefficient can be expressed as thefollowing equation (6):

$\begin{matrix}\begin{matrix}{\frac{h_{11 - 1}h_{11 - 1}^{*}}{{h_{11 - 1}}{h_{12 - 1}}} = \frac{{h_{11 - 1}}^{2}}{{h_{11 - 1}}^{2}}} \\{= 1}\end{matrix} & (6)\end{matrix}$

For example, if a value of 0.5 is used as a predetermined referencevalue, the calculated correlation coefficient exceeds the predeterminedreference value.

Thus, if a calculated correlation coefficient exceeds a predeterminedreference value, the path selection unit 13 determines a path selectionsignal X_(sel) that preferentially selects a path having a lowcorrelation with a path identified by another reception antenna.

First, the path selection unit 13 determines priorities of the receptionantennas 11-1 and 11-2. Here, the first reception antenna 11-1 has ahigher priority. The path selection unit 13 then determines a total setU, a selected path set U₁, and an unselected path set U₂ based on pathincoming times of all the paths. The total set U is a union of sets ofincoming times of paths identified by the reception antennas 11-1 and11-2. The total set U is expressed as:U={τ₁,τ₂}

On the other hand, the selected path set U₁ is an empty set. Theunselected path set is a difference between the total set U and theselected path set U₁. The unselected path set is expressed as:U₂={τ₁,τ₂}

Based on these sets, the path selection unit 13 selects a path for thefirst reception antenna 11-1 having a higher priority from theunselected path set U₂. Here, it is assumed that the path τ₁ isselected.

Next, the path selection unit 13 updates the selected path set and theunselected path set using the selected path τ₁, as follows:U₁={τ₁}U ₂ =U−U ₁={τ₂}

In the same manner as described above, a path is selected from theunselected path set for the second reception antenna 11-2 having thesecond priority. Here, the path τ₂ is selected. Finally, the pathselection unit 13 outputs a path selection signal X_(sel) obtainedthrough the above-described steps.

The de-spreader units 14-1 and 14-2 for the respective receptionantennas 11-1 and 11-2 de-spread received signals based on the pathselection signal and output respective de-spreading signals y₁₋₁ andy₂₋₁. At that time, a de-spreading signal obtained by the firstreception antenna 11-1 is given by the following equation:y ₁₋₁ =h ₁₁₋₁ b ₁ +h ₂₂₋₁ b ₂ +n ₁

On the other hand, a de-spreading signal obtained by the secondreception antenna 11-2 is given by the following equation:y ₂₋₁ =h ₁₁₋₂ b ₁ +h ₂₂₋₂ b ₂ +n ₂

These equations are expressed as the following matrix notation (7):

$\begin{matrix}\begin{matrix}{Y = \begin{bmatrix}y_{1 - 1} \\y_{2 - 1}\end{bmatrix}} \\{= {{\begin{bmatrix}h_{11 - 1} & h_{22 - 1} \\h_{11 - 2} & h_{22 - 2}\end{bmatrix}\begin{bmatrix}b_{1} \\b_{2}\end{bmatrix}} + \begin{bmatrix}n_{1} \\n_{2}\end{bmatrix}}} \\{= {{Hb} + n}}\end{matrix} & (7)\end{matrix}$

The demodulation unit 15 calculates the inverse matrix of the channelinformation H and reproduces a data sequence as follows:b ₀ =H ⁻¹ Y=b+H ⁻¹ n.

Thus, a signal can be correctly received even when the paths havecorrelation.

FIG. 7 is a block diagram of a reception device according to anotherembodiment of the present invention. FIG. 8 illustrates an example of apropagation path model according to the embodiment of the presentinvention. A reception device 3 according to the embodiment of thepresent invention is described with reference to FIGS. 7 and 8.

In FIG. 7, the reception device 3 includes two reception antennas 11-1and 11-2. Two de-spreader units 14-1 and 14-2 and two de-spreader units14-3 and 14-4 are connected to the reception antennas 11-1 and 11-2,respectively. On the other hand, a transmission device (not shown)includes four transmission antennas. Propagation paths between theantennas all include four paths (τ₁ to τ₄) shown in FIG. 8. Here, it isalso assumed that signals transmitted from the transmission antennashave complete correlation.

Accordingly, like the path selection unit 13 of the reception device 2shown in FIG. 6, a path selection unit 13 of the reception device 3detects a correlation value greater than or equal to a predeterminedreference value.

Among priorities of the antennas determined by the path selection unit13, the first reception antenna 11-1 has the highest priority. As in theexample described in relation to FIG. 6, a total set U, a selected pathset U₁, and an unselected path set U₂ are determined as follows:U={τ₁,τ₂,τ₃,τ₄}U₁=0U ₂ =U−U ₁={τ₁,τ₂,τ₃,τ₄}

The path selection unit 13 determines one path for each of the receptionantennas 11-1 and 11-2. When the path selection unit 13 selects a pathfrom the unselected path set, the reception device 3 preferentiallyselects a path having high power.

The path selection unit 13 first selects τ₁ as a path selected for thefirst reception antenna 11-1. Thus, the selected path set U₁, and theunselected path set U₂ are updated as follows:U₁={τ₁}U ₂={τ₂,τ₃,τ₄}

The path selection unit 13 then selects τ₂, which has the highest powerin the unselected path set, as a path selected for the second receptionantenna 12. Thus, the selected path set and the unselected path set areupdated as follows:U₁={τ₁,τ₂}U₂={τ₃,τ₄}

Subsequently, the path selection unit 13 selects τ₃ as a path selectedfor the first reception antenna 11-1 again. Thus, the selected path setU₁, and the unselected path set U₂ are updated as follows:U₁={τ₁,τ₂,τ₃}U₂={τ₄}

Finally, the path selection unit 13 then selects τ₄ as a path selectedfor the second reception antenna 11-2 and outputs a path selectionsignal X_(sel) obtained here. The two de-spreader units 14-1 and 14-2and the two de-spreader units 14-3 and 14-4 connected to the respectivereception antennas 11-1 and 11-2 de-spread input signals using the pathselection information X_(sel) and output de-spreading signals y₁₋₁,y₁₋₂, y₂₋₁, and y₂₋₂, respectively.

Here, by considering that these de-spreading signals have completecorrelation, the de-spreading signals can be expressed as follows:y ₁₋₁ =h ₁₁₋₁ b ₁ +h ₁₂₋₁ b ₂ +h ₁₃₋₁ +h ₁₄₋₁ +n ₁₋₁y ₁₋₂ =h ₁₁₋₃ b ₁ +h ₁₂₋₃ b ₂ +h ₁₃₋₃ +h ₁₄₋₃ +n ₁₋₂y ₂₋₁ =h ₁₁₋₂ b ₁ +h ₁₂₋₂ b ₂ +h ₁₃₋₂ +h ₁₄₋₂ +n ₂₋₁y ₂₋₂ =h ₁₁₋₄ b ₁ +h ₁₂₋₄ b ₂ +h ₁₃₋₄ +h ₁₄₋₄ +n ₂₋₂

These de-spreading signals are also expressed as the following matrixnotation (8):

$\begin{matrix}\begin{matrix}{Y = \begin{bmatrix}y_{1 - 1} \\y_{1 - 2} \\y_{2 - 1} \\y_{2 - 2}\end{bmatrix}} \\{= {{\begin{bmatrix}h_{11 - 1} & h_{12 - 1} & h_{13 - 1} & h_{14 - 1} \\h_{11 - 3} & h_{12 - 3} & h_{13 - 3} & h_{14 - 3} \\h_{11 - 2} & h_{12 - 2} & h_{13 - 2} & h_{14 - 2} \\h_{11 - 4} & h_{12 - 4} & h_{13 - 4} & h_{14 - 4}\end{bmatrix}\begin{bmatrix}b_{1} \\b_{2} \\b_{3} \\b_{4}\end{bmatrix}} + \begin{bmatrix}n_{1 - 1} \\n_{1 - 2} \\n_{2 - 1} \\n_{2 - 2}\end{bmatrix}}} \\{= {{Hb} + n}}\end{matrix} & (8)\end{matrix}$

Therefore, by premultiplying the equation by an inverse matrix of thechannel information H, a data sequence is reproduced as follows:b ₀ =H ⁻¹ Y=b+H ⁻¹ n

As described above, by using a reception device according to the presentinvention, superior communication quality can be obtained even whenpropagation paths have correlation.

The present invention can be applied as in the above-describedembodiment and the other above-described embodiment individually or bycombining the two above-described embodiments.

Also, the present invention can be applied to the followingconfigurations and operations.

That is, a reception device according to the present invention includesn number of reception antennas, where n is an integer greater than orequal to 2. The reception device further includes a path selectiondevice and K_(n) number of de-spreader units for each reception antenna,where K_(n) is an integer greater than or equal to 1, n denotes an nthantenna, and 1≦n≦K_(n).

The reception antennas receive a spread spectrum signal transmitted fromm number of transmit antennas, where m is an integer greater than orequal to 2. The path selection unit inputs the received spread spectrumsignal transmitted from the transmit antennas and selects a path forde-spreading at each de-spreader unit. The path selection unit thenoutputs the selection result as a path selection signal. Each of thede-spreader units inputs the signal received by the reception antennaand the path selection signal and outputs a signal de-spread based onthe path selection signal.

The path selection unit calculates a correlation value of propagationpaths between m number of the transmit antennas and n number of thereception antennas. If the calculated correlation value is smaller thana predetermined reference value, the path selection unit independentlyselects K_(n) number of the paths having a largest power at each of thereception antennas. If the calculated correlation value is larger thanthe predetermined reference value, the path selection unitpreferentially selects K_(n) number of the paths having a lowcorrelation with paths identified by the other reception antennas, ateach of the reception antennas. In this case, the path selection unitemploys the amplitude of a complex correlation value as thepredetermined reference value.

Additionally, the path selection unit determines priorities for n numberof the reception antennas and selects P number of paths for de-spreadingat each reception antenna in accordance with the determined priorities,where P is an integer and 1<P<K_(n). Alternatively, the path selectionunit may repeat this processing until the number of paths identified byeach reception antenna reaches K_(n).

Here, when determining the priorities for n number of receptionantennas, the path selection unit may carry out the followingprocessing:

(1) Detecting a path having a highest power for each reception antennafrom among paths identified by the reception antenna.

(2) Detecting a path having a lowest power for each reception antennafrom among paths identified by the reception antenna.

(3) Calculating an average power of paths identified by each receptionantenna.

In this case, the path selection unit may assign the priorities asfollows:

assigning higher priorities to reception antennas having the detectedpath with higher power values;

assigning higher priorities to reception antennas having the detectedpath with lower power values;

assigning higher priorities to reception antennas with higher averagepower values; or

assigning higher priorities to reception antennas with lower averagepower values.

Furthermore, when determining the priorities for n number of receptionantennas, the path selection unit may carry out the followingprocessing:

The path selection unit calculates the number of paths having a powervalue exceeding P_(th) (P_(th) is any real value) among paths identifiedby each reception antenna for the antenna. The path selection unit thenassigns a higher priority to a reception antenna having more pathshaving a power value exceeding P_(th). Alternatively, the path selectionunit may assign a higher priority to a reception antenna having lesspaths having a power value exceeding P_(th).

Still furthermore, when determining the priorities for n number of thereception antennas, the path selection unit may carry out the followingprocessing:

The path selection unit detects a path having an earliest incoming timeamong paths identified each reception antenna for the antenna. The pathselection unit then assigns a higher priority to a reception antennahaving a path with an earlier incoming time. Alternatively, the pathselection unit may assign a higher priority to a reception antennahaving a path with a later incoming time.

On the other hand, when determining the priorities for n number of thereception antennas, the path selection unit can calculate an averagedelay time weighted by the power of a path for paths identified by eachreception antenna for the each antenna. Then, the path selection unitcan assign a higher priority to a reception antenna having a smallercalculated delay time. Alternatively, the path selection unit may assigna higher priority to a reception antenna having a larger calculateddelay time.

When determining the priorities for n number of reception antennas, thepath selection unit can determine the priorities among the receptionantennas at random.

In addition, when determining the priorities for n number of thereception antennas, the path selection unit can consider a union of setsof the incoming time of paths identified by each reception antenna as atotal set and can determine a selected path set using information aboutselected paths for the reception antenna and the other antennas. Thepath selection unit can consider a difference between the total set andthe selected path set as an unselected path set. Thereafter, the pathselection unit can determine a path used for de-spreading based on theunselected path set. In this case, the path selection unit can select apath used for de-spreading at random.

Furthermore, when determining the selected path set, the path selectionunit can determine samples from F samples before the incoming time ofthe path used for de-spreading at one of the reception antennas to Bsamples after the incoming time as a partial set of the selected pathset, where F and B are integers greater than or equal to 0. Then, thepath selection unit can create a union set of partial selected path setsfor all reception antennas. The obtained set can be used as the selectedpath set.

Still furthermore, when selecting a path used for de-spreading from agroup of unselected paths, the path selection unit can select a pathhaving the highest reception power from among the group of unselectedpaths, can select a path having the earliest incoming time from amongthe group of unselected paths, or can select a path from among the groupof unselected paths at random.

As described above, according to the present invention, a receptiondevice and a radio communication system that has the above-describedconfigurations and operations can advantageously provide superiorcommunication quality even when the propagation paths have highcorrelation.

1. A reception device comprising: n number of reception antennas forreceiving spread spectrum signals transmitted from a transmission devicevia m number of transmit antennas, where m is an integer greater than orequal to 2 and n is an integer greater than or equal to 2; a pathselection unit for selecting a path for de-spreading the signal receivedby the reception antennas and outputting a selection result as a pathselection signal; and K_(n) number of de-spreader units for de-spreadingthe signal received by the reception antennas based on the pathselection signal and outputting the de-spread signals, where K_(n) is aninteger greater than or equal to 1, n represents an n-th receptionantenna, and 1≦n≦K_(n), wherein the path selection unit calculates acorrelation value of propagation paths among the m number of transmitantennas and the n number of reception antennas and wherein, if thecalculation result is smaller than a predetermined reference value, thepath selection unit independently selects K_(n) number of paths having alargest power at each of the n number of reception antennas, andwherein, if the calculation result is greater than the predeterminedreference value, the path selection unit preferentially selects K_(n)number of paths having a low correlation with paths identified by theother reception antennas, at each of the n number of reception antennas.2. The reception device according to claim 1, wherein the amplitude of acomplex correlation value is used as the predetermined reference value.3. The reception device according to claim 1, wherein the path selectionunit determines priorities of the n number of reception antennas,selects P number of paths for de-spreading at each of the n number ofreception antennas in accordance with the determined priorities, andrepeats the selection until the number of paths identified by each ofthe n number of reception antennas reaches K_(n) where P is an integergreater than or equal to 1 and 1≦P≦K_(n).
 4. The reception deviceaccording to claim 3, wherein the path selection unit detects a pathhaving maximum power from among paths identified by each of the n numberof reception antennas for each antenna when determining priorities ofthe n number of reception antennas and wherein the path selection unitdetermines the priorities based on the detection result.
 5. Thereception device according to claim 4, wherein the path selection unitassigns a higher priority to a reception antenna having a detected pathof a higher power.
 6. The reception device according to claim 4, whereinthe path selection unit assigns a higher priority to a reception antennahaving a path of a lower detected power.
 7. The reception deviceaccording to claim 3, wherein the path selection unit detects a pathhaving minimum power from among paths identified by each of the n numberof reception antennas for each antenna when determining priorities ofthe n number of reception antennas and wherein the path selection unitdetermines the priorities based on the detection result.
 8. Thereception device according to claim 7, wherein the path selection unitassigns a higher priority to a reception antenna having a path of alower detected power.
 9. The reception device according to claim 7,wherein the path selection unit assigns a higher priority to a receptionantenna having a path of a higher detected power.
 10. The receptiondevice according to claim 3, wherein the path selection unit calculatesaverage power of paths identified by each of the n number of receptionantennas for each antenna when determining priorities of the n number ofreception antennas and wherein the path selection unit determines thepriorities based on the calculation result.
 11. The reception deviceaccording to claim 10, wherein the path selection unit assigns a higherpriority to a reception antenna having a higher average power value. 12.The reception device according to claim 10, wherein the path selectionunit assigns a higher priority to a reception antenna having a loweraverage power value.
 13. The reception device according to claim 3,wherein the path selection unit calculates the number of paths having apower exceeding P_(th) from among paths identified by each of the nnumber of reception antennas for each antenna when determiningpriorities of the n number of reception antennas, where P_(th) is anyreal value, and wherein the path selection unit determines thepriorities based on the calculation result.
 14. The reception deviceaccording to claim 13, wherein the path selection unit assigns a higherpriority to a reception antenna having more paths exceeding P_(th). 15.The reception device according to claim 13, wherein the path selectionunit assigns a higher priority to a reception antenna having less pathsexceeding P_(th).
 16. The reception device according to claim 3, whereinthe path selection unit detects a path having an earliest incoming timefrom among paths identified by each of the n number of receptionantennas for each antenna when determining priorities of the n number ofreception antennas and wherein the path selection unit determines thepriorities based on the detection result.
 17. The reception deviceaccording to claim 16, wherein the path selection unit assigns a higherpriority to a reception antenna having a detected path with an earlierincoming time.
 18. The reception device according to claim 16, whereinthe path selection unit assigns a higher priority to a reception antennahaving a detected path with a later incoming time.
 19. The receptiondevice according to claim 3, wherein the path selection unit calculatesan average delay time weighted by a power of a path among pathsidentified by each of the n number of reception antennas whendetermining priorities of the n number of reception antennas, andwherein the path selection unit determines the priorities based on thecalculation result.
 20. The reception device according to claim 19,wherein the path selection unit assigns a higher priority to a receptionantenna having a shorter calculated average delay time.
 21. Thereception device according to claim 19, wherein the path selection unitassigns a higher priority to a reception antenna having a longercalculated average delay time.
 22. The reception device according toclaim 3, wherein, when determining priorities of the n number ofreception antennas, the path selection unit determines the priorities atrandom.
 23. The reception device according to claim 3, wherein, whendetermining the path for de-spreading at one of the reception antennasand determining priorities of the n number of reception antennas, thepath selection unit determines a union of sets of incoming times ofpaths identified by each of the n number of reception antennas as atotal set, determines a selected path set using path information aboutpaths selected for the reception antenna and the other receptionantennas, determines a difference between the total set and the selectedpath set as an unselected path set, and determines the path used forde-spreading from the unselected path set.
 24. The reception deviceaccording to claim 23, wherein, when determining the selected path set,the path selection unit determines samples from F samples before theincoming time of the path used for de-spreading at one of the receptionantennas to B samples after the incoming time as a partial set of theselected path set, creates a union of the partial set of the selectedpath set for all of the n number of reception antennas, and determinesthe created set as the selected path set, where F is an integer greaterthan or equal to 0 and B is an integer greater than or equal to
 0. 25.The reception device according to claim 23, wherein, when determiningthe path used for de-spreading from the unselected path set, the pathselection unit selects a path having the highest reception power fromthe unselected path set.
 26. The reception device according to claim 23,wherein, when determining the path used for de-spreading from theunselected path set, the path selection unit selects a path having theearliest incoming time from the unselected path set.
 27. The receptiondevice according to claim 23, wherein, when determining the path usedfor de-spreading from the unselected path set, the path selection unitselects a path at random from the unselected path set.
 28. A radiocommunication system having: a reception device comprising: n number ofreception antennas for receiving spread spectrum signals transmittedfrom a transmission device via m number of transmit antennas, where m isan integer greater than or equal to 2 and n is an integer greater thanor equal to 2; a path selection unit for selecting a path forde-spreading the signal received by the reception antennas andoutputting a selection result as a path selection signal; and K_(n)number of de-spreader units for de-spreading the signal received by thereception antennas based on the path selection signal and outputting thede-spread signals, where K_(n) is an integer greater than or equal to 1,n represents an nth reception antenna, and 1≦n≦K_(n), wherein thereception device selects a path in accordance with a correlation valueof propagation paths; wherein the path selection unit calculates acorrelation value of propagation paths among the m number of transmitantennas and the n number of reception antennas, and wherein, if thecalculation result is smaller than a predetermined reference value, thepath selection unit independently selects K_(n) number of paths having alargest power at each of the n number of reception antennas, andwherein, if the calculation result is greater than the predeterminedreference value, the path selection unit preferentially selects K_(n)number of paths having a low correlation with paths identified by theother reception antennas, at each of the n number of reception antennas.29. The radio communication system according to claim 28, wherein thepath selection unit determines priorities of the n number of receptionantennas, selects P number of paths for de-spreading at each of the nnumber of reception antennas in accordance with the determinedpriorities, and repeats the selection until the number of pathsidentified by each of the n number of reception antennas reaches K_(n),where P is an integer greater than or equal to 1 and 1≦P≦K_(n).
 30. Amethod comprising: receiving, at n number of reception antennas, spreadspectrum signals transmitted from a transmission device via m number oftransmit antennas, where m is an integer greater than or equal to 2 andn is an integer greater than or equal to 2; selecting, using a pathselection unit, a path for de-spreading the signal received by thereception antennas and outputting a selection result as a path selectionsignal; and de-spreading, using K_(n) number of de-spreader units, thesignal received by the reception antennas based on the path selectionsignal and outputting the de-spread signals, where K_(n) is an integergreater than or equal to 1, n represents an n-th reception antenna, and1≦n≦K_(n), wherein the path selection unit calculates a correlationvalue of propagation paths among the m number of transmit antennas andthe n number of reception antennas and wherein, if the calculationresult is smaller than a predetermined reference value, the pathselection unit independently selects K_(n) number of paths having alargest power at each of the n number of reception antennas, andwherein, if the calculation result is greater than the predeterminedreference value, the path selection unit preferentially selects K_(n)number of paths having a low correlation with paths identified by theother reception antennas, at each of the n number of reception antennas.